The present invention relates to an inspection method using an electron beam and an inspection apparatus therefor, and particularly to an inspection method using an electron beam and an inspection apparatus therefor, which are suitable for inspecting patterns of circuits, etc. on wafers in the course of fabricating semiconductor devices.
As an apparatus for inspecting circuit patterns used for a process of fabricating semiconductor devices, lithography masks, reticles, or the like, there has been known an optical inspection apparatus for detecting a defect on a circuit pattern by irradiating the circuit pattern with light and detecting the reflected light with a CCD or the like. The optical inspection apparatus, however, has a limitation in its resolution, and therefore, as the width of a circuit pattern becomes fine, it is difficult to detect a defect on the pattern by the optical inspection apparatus. Accordingly, an inspection apparatus using an electron beam, which has a high resolution, has come to be used for inspecting a defect on a fine pattern.
As one of apparatuses for observing a sample with an electron beam, there is known a scanning electron microscope (hereinafter, referred to as a xe2x80x9cSEMxe2x80x9d). Also, as one of apparatuses for inspecting a semiconductor device with an electron beam, there is known a critical dimension scanning electron microscope (hereinafter, referred to as a xe2x80x9cCDSEMxe2x80x9d). The SEM or CDSEM is suitable for observing a by restricted field of vision at a high magnification; however, it is unsuitable for searching a defect position on a semiconductor wafer. To be more specific, to search a defect position on the semiconductor wafer, it is required to inspect a very wide region, that is, the entire surface region of the semiconductor wafer ranging from 200 mm to 300 mm in diameter, and it takes a lot of time to inspect such a wide region by using the SEM or CDSEM because the electron beam current is low and thereby the scanning speed is low in the SEM or CDSEM. Accordingly, if the SEM or CDSEM is used for inspecting patterns at midway steps of a process of fabricating a semiconductor device, it taken an excessively longer time from the practical viewpoint to inspect the patterns. An inspection apparatus used for inspecting patterns at midway steps of a process of fabricating a semiconductor device is required to speed up the inspection time for increasing the throughput.
An inspection apparatus to solve the above problem has been disclosed, for example, in Japanese Patent Laid-open No. Hei 5-258703, which is configured to detect a defect on a wafer by making use of comparison between images. The inspection apparatus is characterized in (a) using a large electron beam current; (b) continuously moving a sample stage while irradiating a sample or a substrate with an electron beam; (c) using a high acceleration voltage to accelerate an electron beam generated from an electron source; (d) applying a retarding voltage to a sample to decelerate an electron beam, thereby preventing the charging of the sample; and (e) detecting charged particles generated from a sample by irradiation of an electron beam after the charged particles pass through an objective lens, which technique is called a TTL (Through The Lens) type method. The above inspection apparatus makes it possible to more efficiently inspect a defect on a mask or a wafer at a higher speed as compared with the conventional SEM.
In the TTL method, since charged particles generated from a sample are detected after passing through an objective lens, the distance between the objective lens and the sample can be shortened; and also the focal point of the objective lens can be shortened, to reduce aberration of an electron beam, thereby obtaining an image with a high resolution. The TTL method, however, has a non-negligible problem that the rotation of an electron beam largely varies depending on a change in height of a sample, to rotate the obtained image. Accordingly, the TTL method must ensure the accuracy in height of a sample, and therefore, it has a limitation in improvement of the inspecting speed.
The inspection apparatus described in the above document, Japanese patent Laid-open No. Hei 5-258703 adopts a collimated beam for avoiding dimness of the focal point due to Coulomb repulsive interaction of electrons in an electron beam. The adoption of such a collimated beam, however, causes a problem. When a collimated beam is blanked during movement of a sample on a sample stage, part of the collimated beam is not shielded by a stop disposed in a midway point of the trajectory of the electron beam during blanking, whereby a region not required to be irradiated, which is adjacent to a region required to be irradiated, is irradiated with the part of the collimated beam not shield. This results in a possibility that the obtained image is different from the actual one.
FIG. 14 shows a relationship between a retarding voltage and an efficiency of detecting secondary electrons, which is obtained by using a wafer as a sample in a process of fabricating a semiconductor device. In the TTL method shown by (2) in FIG. 14, there occurs a problem that as the retarding voltage is reduced, the efficiency of detecting secondary electrons becomes as low as not to be non-negligible. In the TTL method, secondary electrons generated from a sample are converged through a magnetic field in an objective lens, and the main reason why the efficiency of detecting secondary electrons becomes low as the retarding voltage is reduced is that when the retarding voltage is changed, the irradiation energy given from the electron beam to the sample is changed, with a result that the converged positions of secondary electrons in the axial direction are changed.
To prevent the reduction in efficiency of detecting secondary electrons, it may be considered to increase the retarding voltage; however, if the retarding voltage is increased, since the retarding voltage is applied to a sample stage and a shield frame which surround the end portion of the sample stage is earthed, a discharge occurs between the end portion of the sample stage and the shield frame. This causes an inconvenience in reducing the effect of applying the retarding voltage, or making unstable the electron beam due to occurrence of noise.
Further, since the ease of charge of the sample is dependent on the material of the sample, the magnitude of the retarding voltage must be changed depending on the ease of charge of the kind of sample.
With respect to the irradiation position of an electron beam, the position of a sample stage on which a sample is mounted is accurately measured, and the irradiation position of the electron beam is determined on the basis of the position of the sample stage. An interferometer using a laser beam is provided to measure the position of the sample stage, wherein a laser beam is made incident on mirrors mounted on the sample stage and a minutely changed amount of the position of the sample stage is measured on the basis of the interference of the reflected laser beam. On the other hand, the retarding voltage is applied to the sample via the sample stage on which the sample is mounted, and accordingly, the retarding voltage is also applied to the mirrors mounted to the end portions on two sides of the sample stage. In this case, since the mirror is made from glass, an electric field is concentrated at the end portion of the mirror. As a result, there is a possibility that a discharge occurs between the mirror and another member such as a shield frame provided in proximity to the mirror and earthed. If the mirror is not made from glass, there may occur a discharge between an edge of the mirror made from metal and said another member.
Accordingly, it is required to take into account not only a discharge between the end portion of the sample stage to which the retarding voltage is applied and the shield frame surrounding the sample stage but also the concentration of an electric field at the end portion of the mirror provided for measuring the position of the sample stage.
The above-described discharge phenomenon occurring between the sample stage to which the high retarding voltage is applied and the shield frame can be prevented by sufficiently increasing a distance between the sample stage and the shield frame; however, the increased distance therebetween leads to an increase in the size of the apparatus. In a process of fabricating a semiconductor device, a fabrication apparatus and an inspection apparatus must be disposed in a clean room, and the investment in plant and equipment becomes large in proportional to the floor area of the clean room. From this viewpoint, the inspection apparatus used in the clean room is required not only to realize a higher inspection speed but also to realize space saving by miniaturization.
On the other hand, in the above-described inspection apparatus using an electron beam, as a result of increasing the resolution thereof, the shape in transverse cross-section of the electron beam emitted on a sample may become not a circular shape but a triangular shape with its corners rounded. For example, when a circular-shaped sample is scanned by the triangular-shaped beam, the image on a monitor will be triangular in shape. For another example, when a beam size nearly equals a size of a pixel of the image on the monitor, a foot of the triangular-shaped beam expands the next pixels. That is, the triangular-shaped beam shades off compared with the circular-shaped beam. The above-described inspection apparatus using an electron beam is configured to compare an image derived from a pattern at one location with an image derived from the same pattern at a different location and detect a difference between the two images as an abnormality or a defect. If the shape of the beam is triangular causing expanding, a defect in a fine pattern will not be detected.
The cause of making triangular the shape in transverse cross-section of an electron beam may be considered as follows: namely, since a chip of an electron source, an extraction electrode, a converging lens, and the like are not axisymmetric, the electron beam itself generated from an electron gun becomes triangular; and the electron beam becomes triangular by a converging lens of an optoelectronic system. Accordingly, even by use of an electron beam allowing a high resolution, if there occur the above-described inconveniences, it is impossible to obtain an accurate inspection result.
An object of the present invention is to provide an inspection method using an electron beam and an inspection apparatus therefor, which are capable of enhancing the resolution, improving the inspection speed and reliability, and realizing miniaturization the apparatus.
To achieve the above object, according to the present invention, there is provided an inspection method using an electron beam, including the steps of; applying a voltage on a sample via a sample stage; converging an electron beam on the sample; scanning the sample with the converged electron beam and simultaneously, continuously moving the sample stage; detecting charged particles generated from the sample; and detecting a defect on the sample on the basis of the detected charged particles; wherein a distance between the sample and the shield frame is determined on the basis of a critical discharge between the sample stage and the shield frame. According to the present invention, there is also provided an inspection apparatus used for the above inspection method. In the above method and apparatus, preferably, coils of at least hexapoles for correcting the shape of an electron beam may be provided; the electron beam may be deflected for blanking during movement of the sample with the crossover of the electron beam taken as a fulcrum of blanking; or the magnitude of the voltage applied to the sample may be determined depending on the kind of sample.